The present invention relates to the field of material science and, more particularly, to a method of fabricating at least one single layer hexagonal boron nitride (h-BN).
Boron Nitride
Boron nitride (BN) is a synthetic material fashionable in both hexagonal and cubic structures (See K. Watanabe, T. Taniguchi, and H. Kanda, Nature Mater. 3, 404 (2004), T. Taniguchi, K. Watanabe, and S. Koizumi, Physics Status Solidi A. 201, 2573 (2004), V. A. Gubanov, Z. W. Lu, B. M. Klein, and C. Y., Physical Review B 53, 4377 (1996.). Hexagonal BN (h-BN) consists of sp2-bonded two-dimensional (2D) layers comprising alternate boron and nitrogen atoms in a honeycomb arrangement; these layers are stacked and van-der-Waals bonded to form a highly anisotropic three-dimensional (3D) crystal. The overall structure and atomic spacings of h-BN are very similar to carbon-based graphite (See A. Rubio, J. L. Corkill, and M. L. Cohen, Physical Review B 49, 5081 (1994), N. G. Chopra, R. J. Luyken, K. Cherrey, V. H. Crespi, M. L. Cohen, S. G. Louie, and A. Zettl. Science 269, 966 (1995).).
In h-BN, however, the boron and nitrogen atoms are alternately stacked directly on top of each other on the adjacent atomic layers resulting in AAA stacking, as shown in FIG. 1, while graphite maintains an offset Bernal structure (ABA). Boron nitride maintains an AAA stacking where boron and nitrogen atoms are alternately stacked on top of each other. In addition, the slightly ionic bonding (both in-plane and out-of-plane) in h-BN further sets this material apart from graphite. h-BN is electrically insulating with a large band gap both within and across the layers, while graphite is a semi-metal with high levels of conductivity within the layers (See K. Watanabe, T. Taniguchi, and H. Kanda, Nature Mater. 3, 404 (2004), K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos and A. A. Firsov, Nature 438, 197 (2005), K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004).).
Graphene
Graphene is a single layer of carbon atoms bonded together forming a honeycomb structure. The recent successful isolation and atomic scale investigation of single layer graphite (i.e., grapheme (See K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004), K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, PNAS 102, 10451 (2005), J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, D. Obergfell, S. Roth, C. Girit, and A. Zettl, Solid State Communications 143, 101 (2007), J. C. Meyer, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, T. J. Booth, and S. Roth, Nature 446, 60 (2007).)) has stimulated interest in atomically thin sheets of other layered materials, including BN (See K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, PNAS 102, 10451 (2005), D. Facile, J. C. Meyer, . Ö. Girit, and A. Zettl, Appl. Phys. Let. 92, 133107 (2008).). Single layer h-BN is considered the thinnest possible 2D crystal with slightly ionic bonds. This characteristic makes atomically thin h-BN an ideal model system in which to study atomic configurations, including defects, edges, and vacancies of 2D ionic crystals. Of particular interest is the possibility of using an atomic resolution probe to unambiguously identify the atomic species, i.e., to distinguish boron from nitrogen in any particular layer of h-BN.
Mechanical Exfoliation
The same mechanical exfoliation methods used to isolate graphene from graphite can also be applied to h-BN (See K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, PNAS 102, 10451 (2005), D. Pacilé, J. C. Meyer, . Ö. Girit, and A. Zettl, Appl. Phys. Let. 92, 133107 (2008).). However, due to the stronger interplane bonding in h-BN, mononlayer sheets of h-BN are difficult to isolate, and at best few-atomic-layer specimens are obtained (See D. Pacilé, J. C. Meyer, . Ö. Girit, and A. Zettl, Appl. Phys. Let. 92, 133107 (2008).).
Characterizing Atomic Structure, Defects, and Edges in Thin Free-Standing Membranes
An ideal tool with which to characterize atomic structure, defects, and edges in thin free-standing membranes is a high resolution TEM. Conventional TEMs, however, do not have the required resolution for distinguishing atoms in h-BN and are often operated at very high voltages leading to immediate sample damage before any reliable observations can be made.
Distinguishing Boron from Nitrogen
Boron and nitrogen have similar atomic numbers and core electron configurations, hence similar scattering power for TEM imaging electrons. The resulting intensity profiles for individual B and N atoms in the reconstructed phase image for a monolayer region are thus expected to be similar, but not identical. To distinguish B from N requires the necessary resolution and sensitivity (signal-to-noise).
TEM Instrumentation Errors
Potential TEM instrumentation errors can easily lead to erroneous image intensity results and the misidentification of the atomic sublattices in thin h-BN specimens. One error is sample tilt, where the TEM imaging electron beam is not perfectly normal to the sample layers. Another is coma astigmatism associated with the TEM electron optics.
Imaging and Atomic Structure of Boron Nitride
C. Jin, F. Lin, K. Suenaga, and S. Iijima. Phys. Rev. Lett. 102, 195505 (2009) and J. C. Meyer, A. Chuvilin, G. Algara-Siller, J. Biskupek, and U. Kaiser, Nano Lett., DOI: 10.1021/N19011497, (2009) studied the imaging and atomic structure of boron nitride. In both studies, suspended atomically-thin layers of h-BN were produced using e-beam irradiation within the TEM itself rather than independently ex-situ. An atomically resolved lattice and triangular defects were observed in both studies.
Meyer et al. were unable to distinguish B from N. Jin et al. used intensity profiles in phase contrast images to distinguish B from N. However, their adjacent atom (column) asymmetry for an identified n=2 bilayer exceeds their asymmetry plotted for an identified n=1 monolayer.
Therefore, a method of fabricating at least one single layer hexagonal boron nitride (h-BN) is needed.